A cathode ray tube is now widely used as an electric display apparatus because of its high speed and ease of scanning. Cathode ray tubes, however, are bulky and may not be practical for portable devices such as portable computers. Furthermore, high voltages are required for operating cathode ray tubes which render such tubes inconvenient for portable devices. It is also difficult to produce a cathode ray rube having a flat display panel. Thus, efforts have been made to develop display devices based upon concepts other than the cathode ray tube.
Transportation of charged particles by means of an applied electric field is commonly known as electrophoresis. Non-emissive display devices based upon electrophoretic principles offer the potential of considerably increased viewability (brightness, contrast and viewing angle) over emissive display devices such as electroluminescent devices.
Electrophoretic image display (EPD) devices have been proposed as alternatives to the cathode ray tube. An EPD device typically comprises a suspension of pigment particles in a dielectric liquid held between two electrodes, at least one of which is transparent. The pigment particles are charged either positively or negatively. The potential difference applied across the electrodes causes the particles to move towards one or the other electrode. If the pigment particles and the dielectric liquid are of two contrasting colors, the movement of the pigment particles will cause images to be displayed which are visible through the transparent electrode or electrodes. EPD-type displays are disclosed in U.S. Pat. Nos. 3,668,106; 3,892,563; 4,041,481; 4,093,534 and 4,203,106. These patents are hereby incorporated by reference to provide the basic information and background on the chemistry and technology of EPD-type displays.
Although the concept of a flat-panel, non-emissive electrophoretic display device is particularly promising for many applications, there have been problems in realizing commercially successful display devices based upon electrophoretic principles. Among the problems have been difficulties in adequate response time, contrast and stability.